since the firmware in my ebike engine Bosch performance CX gen 4 has been updated, I am noticing a strange beahviour, that is, when pedalling, the engine (or the rear wheel pawns) will not engage immediately. Similar to this. Sort of a gap before the engine kicks in. The issue will show in a very perceivable manner when riding:

  1. When the bike is moving already, you stop pedalling
  2. The bike keeps moving
  3. You start pedalling again - this is when the defect is perceivable

Now, I am not sure 100% if this was happening before, as the firmware update, which added more torque, might enhance a beahviour which wasn't initially perceivable with less torque.

So I went to an authorised Bosch service support, and they got in contact with Bosch in Germany, who asked them to make some tests, but from their point of view there's no issue on the engine. The man at the service thinks that the issue could be due to either:

  • A different freewheel pawls number in the rear freehub and the engine ones
  • A low freewheel pawls number (31) in the rear freehub. As for this, the man said that freewheel pawls in professional grade freehubs can go up to 400, ensuring the "grip" is immediate without any gap when you begin pedalling.

I really don't understand why this "gap" was not noticeable before the engine firmware upgrade, but I am quite unhappy as for this feeling...

The chain has been replaced two weeks ago after 1400Km, and it's difficult to say id this defect happens when the engine is off. The bike shift is a derailleur.

  • Does it happen with the engine shut off? does the chain slip on the rear? It maybe the issue is the cassette and/or chain wear, not the engine.
    – EarlGrey
    Commented Jan 12, 2021 at 11:53
  • Not sure, I should further investigate this. But the chain is brand new (recently replaced)
    – Riccardo
    Commented Jan 12, 2021 at 12:57
  • If the chain is brand new, it can still be that that the slippage is due to the cassette. Do you have a derailleur or internal hub gears? It should be possible to reproduce the issue by pushing hard from standing still (for example starting uphill) ... but pay attention to the fact that when the chian slips, the pedals will turn freely ... and your crotch will likely slam on the top tube :( ! Anyhow, checking for cassette wear is rather difficult, it may be that only some gear on the back are worn out, not all of them, is this defect happening with every gears on the back?
    – EarlGrey
    Commented Jan 12, 2021 at 13:09
  • The bike is equipped with a derailleur. It looks more prone to the issue in lower gears
    – Riccardo
    Commented Jan 12, 2021 at 14:45
  • 1
    What 'mode' are you riding in? The 2020 firmware update absolutely does change/update the assist algorithms, in addition to 'more torque.' So eMTB mode especially feeling a bit different than it did before is not surprising.
    – Affe
    Commented Jan 12, 2021 at 20:39

3 Answers 3


The 2020 Bosch performance CX firmware update does change the algorithms for adaptive support in addition to increasing low-cadence torque. It intentionally 'ramps up' the amount of assist more slowly than it did before when starting out/pedal pressure is low to get a more natural "bike that helps you" and less "pushy" feel.

  • To be honest the previous firmware version had 75nm in the initial load whilst the new one has 85nm. IMHO it looks unnatural as when the engine kicks in, the human effort is too low. Might need to get used to it
    – Riccardo
    Commented Jan 13, 2021 at 6:56
  • It really depends on your riding style I guess, There aren't a whole lot of times you should really experience noticeably more torque for regular road riding I'd think. The max assist level is still a power multiplier and is unchanged, the torque curve is just a bit flatter basically as far as I can tell so you can achieve max assist across a broader range of cadences. (I am not a Bosch engineer of course, just another Bosch rider.)
    – Affe
    Commented Jan 13, 2021 at 18:39

It’s definitely just your freehub’s slow engagement. The old firmware might have had a smoother introduction of motor power at the expense of responsiveness. The new one might be snappier, which means you’ll feel the dead space more. This problem will manifest itself more in the lower gears because you have to pedal more to rotate the rear wheel the same amount.

Edit: Thanks to Affe, I’ve learned that I got it backwards. The new update is in fact more gradual, so I presume there’s more of a delay before the motor fully kicks in. Your legs will be the primary power source until then, so you’ll feel the freehub delay more clearly.

Depending on what rear hub you have, you may or may not be able to add more pawls for a faster engagement. If your hub is incompatible, you’ll need a fancier one.

  • what kind of test could I do to proof this? And how would I know if the rear hub might host more pawns?
    – Riccardo
    Commented Jan 12, 2021 at 22:05
  • @Riccardo 1: Shift into the easiest gear and count how many clicks you hear in a full revolution of the cranks. 2: You’ll need to search online for your hub model specifically. If you can find the model for me, I can try to do the research for you.
    – MaplePanda
    Commented Jan 13, 2021 at 1:30
  • This is the rear hub...photos.app.goo.gl/oKpexemsNEJhoDMd9
    – Riccardo
    Commented Jan 14, 2021 at 11:38
  • 50 clicks on the engine, 34 on the rear hub
    – Riccardo
    Commented Jan 14, 2021 at 11:47
  • 1
    See this video! It must be at least 12°/15° gap. Pump up your volume to hear the "clank" when the rear hub engages... photos.app.goo.gl/XuTkdee9Rbke5qRQ7
    – Riccardo
    Commented Jan 14, 2021 at 11:56

Since your bike has a derailleur and you recently changed the chain, I am guessing that the cassette is worn out as well, especially in some gears (it may be the case if you often keep the lower gears because the motor bail you out of steep hill & co, when you pedal at a much lower cadence, too slow if you did not have a motor, but you can keep going because the motor is there to support you).

I read what you describe as the chain not engaging immediately on (some worn-out teeth of) the cassette, probably because of the additional torque released by the motor post-firmware. So teh chain slips and to you it feels like the motor did not engage. However, this is a mechanical issue of the cassette, not of the motor, teh motor feels the chain gliding smoothly so it does not apply any torque ... but the issue is the chain not hanging on the rear gears.

It is a well known musical phenomena of bikes without motor: you stop, you get ready to start, you prepare your strong side to have the pedal in the upper position, you push hard with that leg, suddenly the chain slips, the pedal rotates quickly, the foot belonging to the strong leg lowers suddenly and your crotch hits the top tube, music and religious invocations start filling the air.

In short: you should go to the bike shop that changed the chain and ask why they didn't check the rear cassette.

  • 1
    The man who serviced the bike said that the cassette is not worn out
    – Riccardo
    Commented Jan 14, 2021 at 11:59

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